Green gas = propane, silicone oil and perfume stored in unsafe container and marked up in price about 400%.

co2 is, well, it's co2.

The difference in the two (propane and c02) is that c02 is under higher pressure and is thus more powerful. It's also harder on the gun. And it's somewhat less affected by cold tempratures. It's the more expensive of the two as well. Propane can be used in place of green gas with the addition of a cheap ($24.95) adapter kit, is cheaper then both green gas and co2. Propane is what the bulk of GBB's run on. Gas guns that are made for propane will not run on co2, guns that run on co2 can often run on propane if you get propane mags for them.

Kuro_Neko

edit: I'll second the recommendation for KJW pistols. I have an M9 and a 1911 and I love them dearly. KJW guns have a rep for having finicky mags but I haven't had any trouble with any of the mags I have.